In a distributed storage system, data is duplicated and the duplicated data is stored in plural storage systems that are dispersively disposed in order to improve the performance of the distributed storage system by distributing I/O processing and to protect data against failures. In the distributed storage system, there is a problem in that how to deal with the duplicated data when the duplicated data has to be updated. If the synchronization among updated data is performed throughout the entirety of the distributed system, although the duplicated data can be updated with the consistency of the duplicated data being ensured, there arise different latencies for the synchronization among individual distributed storage systems, so that the write performance (at the update time) is sacrificed.
Therefore, Patent Literature 1 discloses a scheme for improving the I/O performance of the distributed storage system by switching a consistency control scheme in accordance with policies.